A two-step process of uterine stromal cell differentiation (sensitization, decidualization), termed decidualization, is required for the success of embryomaternal interrelationships. Sensitization describes the process by which uterine stromal cells acquire the ability to respond to stimuli which initiate the structural and functional changes that characterize the terminally differentiated decidual cell. Sensitization is an obligatory prerequisite for decidual cell transformation. Progesterone is the major determinant in both sensitization and decidualization. Although data suggests that progesterone regulates these processes on the level of transcription actually little in known about the regulatory mechanisms directed by progesterone; particularly at the molecular level. The focus of this proposal is to identify, isolate and characterize those gene(s), activated by progesterone, which regulate the differentiation of the neutral to the sensitized uterine stromal cell. In order to acheive this goal we will construct a wavelength10 cDNA library representative of maximally stimulated uterine stromal cells. The library will be screened with subtracted and differential cDNA probes. cDNAs associated with uterine sensitivity (PX3 cDNAs) will be characterized by Northern RNA blot analyses and nucleotide sequencing. Further characterization, directed by the results of matching the PX3 cDNAs with a computer database, will include restriction mapping, further sequencing, hormone specificity, cell, tissue, species specificity, immunobiology, localization and biological activity. Availability of characterized PX3 (sensitivity) cDNAs will contribute significantly to our understanding of the regulatory processes by which cells become competent to differentiate. This applies directly to our knowledge of implantation (success of in vitro fertilization/embryo transfer programs) and the hormonal regulation of normal and abnormal growth and differentation of cells of mesenchymal lineage (new therapeutic strategies for stromal tumors).